In recent years, nitride semiconductors containing nitrogen as a group V element have been in the limelight in the field of semiconductor light-emitting elements utilizing pn junction, such as light-emitting diodes (LEDs) and laser diodes (LDs), and have been researched and developed. The nitride semiconductors such as AlN, GaN, and InN are direct transition semiconductors. Moreover, nitride semiconductors of ternary mixed crystal type or quaternary mixed crystal type can emit light from infrared light to deep UV light by appropriately setting composition to vary a band gap. In particular, since the UV range is an unexplored light range, further research and development of such nitride semiconductors have been expected.
Wide applications of semiconductor deep UV light sources (LEDs/LDs) with wavelengths of 220 to 350 nm for sterilization/water purification, in the field of the medical/biochemical industries, and the like are expected for the future. The achievement of such applications has been awaited. In addition, such light sources can be used in a wide range of applications for high-density optical recording light sources, white illumination lamps, UV-curable resins, and the like for industrial use, sensing technology such as fluorescence analysis, high-speed decomposition processing of environmentally hazardous substances (e.g., dioxin, endocrine disrupters, and PCB) with the combined use with titanium oxide, and the like. The above light sources are known to have sterilization effects that can be intensified to a maximum at wavelengths of about 260-280 nm overlapping the DNA absorption wavelength. The market size for semiconductor UV light sources is expected to exponentially expand with progress in high efficiency technology. Thus, development of high-efficiency/high-power UV LEDs/LDs is an important subject.
Conventional UV light sources are limited to gas-solid UV light sources such as excimer lasers, argon ion SHG lasers, and excimer lamps. Since they are large-size, short-life, and expensive UV light sources, applications thereof for general purposes have been difficult. If semiconductor UV LEDs/LDs that can replace conventional UV light sources are realized, they will serve as ultracompact, high-efficiency, high-power, long-life, and low-cost UV light sources, compared with gas-solid light sources, leading to the opening of a wide range of application fields. Such semiconductor UV LEDs/LDs will be in high demand in such application fields. Thus, the development of nitride AlGaN-based deep UV light sources is particularly important for the future.
AlGaN-based materials can be selected as materials for realization of UV light-emitting elements. The band gap energy for AlGaN-based materials ranges from 3.4 eV for GaN to 6.2 eV for AlN; such range covers the UV light-emitting regions of various conventionally used gas lasers. In addition, AlGaN-based materials have, for example, the following features: 1) they are direct transition-type semiconductors throughout the entire composition range; 2) they allow high-efficiency UV light emission from quantum wells; 3) p- or n-type semiconductors can be formed with them; 4) they are rigid materials with long element lives; and 5) they are environmentally safe materials free from harmful substances such as arsenic, mercury, and lead. Due to the above reasons, AlGaN-based materials are most promising materials for realization of practical UV light-emitting elements.
As a result of the development of AlGaN-based UV light-emitting devices over roughly the past 15 years, there has been progress; however, the efficiency thereof currently remains at about 1%, which is lower than that of blue LEDs (80% or higher) and the like under the present circumstances. Reduction of the threading dislocation density in an AlN underlayer was a key factor for realization of UV LEDs. In recent years, however, the dislocation density has been reduced to about 1/100 that the previous level, resulting in the improvement of internal quantum efficiency from 0.5% or less to about 50% for 220- to 320-nm AlGaN. Further, the internal quantum efficiency has been improved to about 80% by mixing In in with AlGaN (see, e.g., Patent Literature 1).
Nevertheless, the p-type concentration in AlGaN is still low, causing the efficiency for electron injection into a light-emitting layer to remain at a low level of about 10% to 30%. In addition, the light extraction efficiency for UV LEDs is as low as 6% to 8% due to UV light absorption in the vicinity of a contact layer/electrode. The external quantum efficiency for UV LEDs determined by multiplying the above factors is as low as about 1%, which should be improved in future research.
In particular, it has been long believed that achievement of high electron injection efficiency would be impossible because p-type AlGaN has limited physical properties, making it impossible to improve the hole concentration of p-type AlGaN. For the present invention, the present inventors introduced multiquantum-barrier electron-blocking layers into AlGaN- or InAlGaN-based UV LEDs, thereby successfully improving the electron injection efficiency from 10%-30% to an estimated level of 80% or more by experiment. Accordingly, the present inventors have suggested and demonstrated a method for solving fundamental problems associated with electron injection, which are derived from the “impossibility derived from p-type AlGaN.” The disclosure of the present application relates to the introduction of multiquantum-barrier electron-blocking layers for the improvement of electron injection efficiency for nitride UV light-emitting elements and teaches standards for designing multiquantum-barrier electron-blocking layers, analytical values of the actual effects of such introduction, practical implementation of the present invention for deep UV LEDs, and realization of the world's highest output.